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Week 12


molding and casting



Assignment

  • design a 3D mold, machine it, and cast parts from it


Week workflow



STEPS


Defining the part


I've been working with a friend, Mauricio Cordoba, on his 1 dollar water filter project : FairCap.

The Faircap project webpage

The Faircap project webpage


FairCap was one of the 12 projects selected to participate in POC21.

FairCap at POC21

FairCap at POC21


Now the idea is to prototype a filter that adapts to plastic bottles and small pipes allowing at the same time the use of a new unexpensive porous tubing technology.

The new filter

The new filter


We'll mold and cast this part, the "multi-adapter" :

The multiadapter part

The multiadapter part


The porous tubes :

Porous fibers

Porous fibers that filter at sub virus size with great flow


Here is a section of the whole system :

Section of the filter

Section of the filter


Again, we'll focus on the side multi-adapter

Here is our part to cast :

multi-adapter part

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Designing the molding and casting strategy


Before jumping on the design I read this guide, the guerilla guide to CNC machining.

Guerilla guide index

Guerilla guide index


Aside from the information that was given to us from the fabacademy course, there is some great explanations in it, like this one :

Molding and casting strategy

Molding and casting strategy


Also, doing the Solidworks tutorial on molding was good :

Soliworks Tutorial

Soliworks Tutorial


A great tool in Solidworks is the drafting analysis :

Drafting analysis tool

Drafting analysis tool


It allows us to see how the demoulding would go, and then help us doing slight angles on either part of the split between the two mold sides.

After tweaking the part, here is what shows the drafting tool :

Final draft

Something that we also have to take into consideration when we design the mold is the angle that we need to make on the vertical faces for milling the deeper surfaces

Indeed, the bits need to have room to go down without colliding on other faces of the part.

Angle limit induced by tool shape

Angle limit induced by tool shape


In Solidworks, I do a quick sketch to find the angle based on measurements on the bit.

Finding the angle in Solidworks

Finding the angle in Solidworks


Going mentally through the process of demolding, I realize that the threads that are present on both side fo the part will make it impossible to demold by sliding with hard material for mold.
Not only this, but machining the thread with 3axis CNC seems to be impossible. We need to cast a mold part.

So two ideas :

  • We could unscrew a hard mold part from the final cast part.
  • Or we could cast this mold part with silicone and squish it to take it out.

I choose to go with the Silicon idea :

The complete mold

The complete mold


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Drawing the different parts


So here is an exploded view of the parts I need to make :

Exploded view of the mold

Exploded view of the mold


The silicone parts are mold parts that will be removed by squishing them and pulling them out.

Silicone Negative of side 1

Silicone Negative of side 2

I'm drawing positive parts that are simpler versions of the final part.

Positive parts for casting silicone mold

Positive parts for casting silicone mold


They will be used as positive molds for casting the silicon negative mold :

Positive parts in place

Positive parts in place


Then I will remove them and I'll be left with just the mold parts I need to cast the final part.

Exploded view of the silicone + wax mold

Exploded view of the silicone + wax mold


I just have to put the two sides together.

The mold closed

The mold, closed


I give a 0.1 mm play for the right faces to be tight, not all of them :

Play for getting tightness only on the closing mold faces

Play for getting tightness only on the closing mold faces


I also make channels and vent for injecting the liquid plastic.

One side of the hard wax mold

One side of the hard wax mold


I guess it's better to fill from the bottom to the top to prevent trapped air.

injection channel and vent

injection channel and vent


Same thing for the other side :

The other side of the hard wax mold

The other side of the hard wax mold


Here is a video of how the mold works :

How the mold works

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Making the mold parts


I start by 3Dprinting my positive parts (the black ones on the picture shown earlier).

One for the "green" side :

Positive part - side 1 - in Cura

Positive part - side 1 - in Cura


And for the red side :

Positive part - side 2 - in Cura

Positive part - side 2 - in Cura


The prints :

The printed part

The printed part 1


The printed part 2

The printed part 2


Then I generated the toolpath for machining the hard wax mold part.

For this, I need to export my geometry. A convenient thing to do is to create a coordinate system that outputs our part in the right orientation and position according to the software we use to generate the toolpath.

Setting a new coordinate system for export

Setting a new coordinate system for export


I use Rhino and RhinoCam to generate the toolpath.

mold part imported in Rhino

mold part imported in Rhino


Here are the machining jobs :

The machining strategy

The machining strategy


And the toolpaths that were generated with their simulation :

horizontal roughing

Horizontal Finishing

Parallel Finishing

Parallel Finishing 90°

Just to check I also played with fabmodules :

Fabmodules gcode for milling wax

It looks alright, I like how it's very simple to use.

I first need to cut a piece of wax that fits the mold part that I will mill :

Cutting pieces of wax

Cutting pieces of wax


Then I position it in our Monofab milling machine :

Positioning the wax piece

Positioning the wax piece


Here is a video of the first machining job, roughing :

Roughing operation

The result :

Result of the roughing operation

Result of the roughing operation


The second operation is horizontal finishing. Here is the result :

Result of the horizontal finishing operation

Result of the horizontal finishing operation


Then we have the two parallel finishing operations.

On one of the mold parts, I also generated a toolpath and milled a channel with a 1/32" bit for injecting the plastic.

Here are the two parts at the end of the process :

The two hard wax mold parts

The two hard wax mold parts


The mold closes perfectly :

Closed Mold

Closed Mold


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Casting the desired part


I pour the silicon in one of the mold :

Closed Mold

Legend


I obtain the internal silicone negative :

Closed Mold

Legend


As wished, I'm able to take out the silicone negative off of its 3D printed positive counterpart :

Closed Mold

Legend


This Silicone part will then be the internal negative for our multiadapter.

I do the same for the other side :

Closed Mold

Legend


Now, I just need to cast liquid plastic with these silicone internal negatives at the center of the two parts mold.

I didn't have the time to do this but you can check my final project page for a similar casting.

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Source files

Here are the sources files of the projects I talked about on this page :


SolidWorks files

If you feel like contributing, please fork from Github :



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Conclusion


...




***

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